1. Field of the Invention
This invention relates to in-orbit testing of communication satellites, and more particularly to in-orbit repeater testing from a single location on the ground of a satellite that broadcasts multiple downlink beams over a large geographic area.
2. Description of the Related Art
Multi-beam satellites are tested after a successful launch to verify the operation of the receive and transmit antennas and the electronic subsystems. A satellite repeater includes all of the electronics between the receive and the transmit antennas. The transponder includes a receive antenna, the electronics associated with the antenna, and one or more transmit antennas. As shown in FIG. 1, a single ground station 2 transmits an uplink signal 4 that contains broadcast information. A satellite 6 boosts the signal and distributes it to a plurality of narrow-beam transmit antennas that transmit spot beams 8a-8d to different geographic regions 10a-10d. The use of multiple downlink beams having smaller areas of coverage allows the broadcast information to be transmitted to individuals in specific areas, and reusing those same frequencies in other diverse areas.
In-orbit satellite test procedures are described in Egly et al, "In-Orbit Test of the First Hughes United States Direct Broadcast Satellite", International Journal of Satellite Communications, Vol. 13, No. 5, September-October 1995, pages 301-312 and in Kasstan et al, "Recent Advances in `In-Orbit Testing` of Communications Satellites", International Journal of Satellite Communications, Vol. 13, No. 5, September-October 1995, pages 377-401. The tests disclosed in the articles do not sample the amplified downlink signal prior to transmission from the satellite.
A network of ground test stations are distributed throughout the different geographic regions 10a-10d to receive the downlink signals and verify operation of the satellite subsystems. A test signal is transmitted from ground station 2, amplified, and rebroadcast in downlink beams (8a-8d), which are sampled within their respective areas of coverage (10a-10d). In the example shown in FIG. 1, four test stations, one within each downlink beam, are required to receive downlink signals corresponding to the uplink test signal. Usually the ground test stations are portable so that the downlink signal can be measured from multiple points within the area of coverage. The power tests are conducted from a location within the downlink beam where the power level of the downlink signal is at or near maximum. The measured data is then combined with characteristics of the test signal and processed to verify operation.
A major disadvantage of the test method just described, is that the test is repeated at multiple ground test stations. As the number of channels increases, so does the number of tests which must be performed. Increasing the number of tests performed increases the time and cost of verifying the operation of the satellite.